1. Field of the Invention
The present invention relates to an apparatus for detecting internal pressure of a can, and more particularly to an apparatus for detecting internal pressure suitable for detecting internal pressure of a can formed of a relatively soft material for cans such as aluminum cans, plastic cans and the like.
2. Description of the Prior Art
For the purpose of saving material and reducing cost, there have been recently widely used cans formed of a soft material such as metal cans formed of a thin metal material such as aluminum, plastic cans, etc., as cans for beverages and other foods. These cans are low in strength. Therefore, in the case of carbonated beverages, carbon dioxide gas is sealed into a can, and in the case of non-carbonated beverages, liquefied inert gas such as liquid nitrogen is sealed into a can to generate internal pressure thereby maintaining the strength of a can. For this reason, in the can producing line using the aforementioned can materials, it is particularly important in terms of quality control to conduct inspection if internal pressure of a can is properly secured.
In the past, as a method for measuring internal pressure of a can of this kind, a method for detecting internal pressure of a can has been known which utilizes the fact that a forced amount of a can body and reaction are in a correlation therebetween in a specific range according to internal pressure of a can whereby a can is slightly forced to measure reaction at that time to detect internal pressure of a can. However, the can body is not always circular in section completely due to an error in manufacture, and therefore, in such a case, even if the can body is pressed with the same forced amount, the substantial forced amount is sometimes differentiated to produce an error in measurement. To overcome this problem, the present inventors have proposed an apparatus in which a can body is transported without being rotated between two rolls which are slightly narrower than the outside diameter of the can body and arranged in a slightly different spaced relation, the can body is forced with different amounts of force at the same location, the reaction is measured by each of two load cells, a difference between measured values thereof is converted into an internal pressure to detect internal pressure of a can. (Japanese Patent Laid-Open No. 157,537/1984). The aforementioned apparatus is based on the principle in which the proportional relation of the forced amount of the can body and the reaction is that the higher the internal pressure of a can, the greater the proportional coefficient, and therefore, if the forced amount of the can body at the same location is varied to obtain the respective reaction and the difference thereof is converted into the internal pressure, the error in measurement caused by the error in manufacture is offset even if an error in manufacture is present in the can body whereby accurate internal pressure can be measured.
The measuring apparatus as described above has a detector for displacement of an end cover. This detector for displacement of a lid provided on the apparatus reads detection data at the same time-intervals from said detector during the passage of a can under the detector, compares and arithmetically operates average data at a seam band portion and data in the central portion, and detect a difference between a line connecting upper surfaces of both seam bands for the end cover and the central portion of the cover to thereby measure a displacement of the cover.
The apparatus as proposed above is made possible to detect internal pressure with high accuracy in the wide range of a combination of detection of internal pressure by reaction and a displacement type internal-pressure detector exhibiting a good resolution at a high pressure portion.
However, the apparatus as proposed above has no adjusting means which can simply respond to a change of kind of a can to be measured.
In addition, the detection for displacement of a cover was provided to detect a difference between the line connecting the upper surfaces of the seam bands in the outer periphery of the cover and the central portion of the lid, and therefore, in the case of the easy-open can, it is affected by the direction of an open tab. Furthermore, in detection of reaction of the aforementioned detector, as shown in FIG. 8, a rotating roll 26 is pressured against a can body, and reaction transmitted through a rotational shaft 27 is detected by a load cell 19. Therefore, the aforesaid detector requires a mechanism for maintaining the rotational shaft of a roll in a vertical attitude and a slide mechanism for transmitting a load to a load cell without deflection, and a spline shaft 28 is employed to meet such demand. As the result, in said detector, the weight of the slide portion increases; when the overhung increases, smooth slide operation is difficult to obtain; and the slide resistance varies. Furthermore, since the processing accuracy of the rotational portion is normally approximately 5/100 (mm), when the can is pressed by the rotational mechanism, a difference of 0.05 at the maximum occurs in the forced amount. The distance between the gauge surface of the load cell and the contact point increases, and the expansion of parts due to heat cannot be ignored. These factors result in an error in value of measurement, influencing on the discriminating accuracy.